The fabrication of composite articles comprising particulate ceramic materials bonded with a glass-ceramic has been practiced for a number of years. One example of that practice is illustrated in U.S. Pat. No. 4,861,734 (MacDowell), which patent described the manufacture of glass-ceramic bodies through the sintering of glass powders into an integral body, the glass powders essentially simultaneously crystallizing in situ during the sintering process. The precursor glasses disclosed therein were selected from the following three composition families, the recited ranges being expressed in terms of weight percent on the oxide basis:
(a) 20-30% CaO, 35-55% Al.sub.2 O.sub.3, and 20-40% B.sub.2 O.sub.3 ; PA1 (b) 30-45% SrO, 30-45% Al.sub.2 O.sub.3, and 20-35% B.sub.2 O.sub.3 ; PA1 (c) 40-55% BaO, 25-40% Al.sub.2 O.sub.3, and 15-30% B.sub.2 O.sub.3 ; and PA1 (d) mixtures thereof.
The patent noted that, because the glass powders crystallized so rapidly during the sintering process, the disclosed glass powders were suitable for sealing operations, but their viscosity characteristics militated against their widespread utility in forming glass-ceramic-bonded ceramic composites.
A second example of the fabrication of composite articles comprising particulate ceramic materials bonded via a glass-ceramic is disclosed in U.S. Pat. No. 5,112,777 (MacDowell). That patent referred generally to glass-ceramic-bonded refractory materials, noting the applicability of such diverse materials as Al.sub.2 O.sub.3, cordierite, SiC, Si.sub.3 N.sub.4, TiN, and ZrO.sub.2. The patent proposed the inclusion of such highly refractory particulates as Al.sub.2 O.sub.3 and/or ZrO.sub.2 as fillers in glass-ceramic coatings to provide barriers on various substrates to attack by hydrogen and oxygen. The patent emphasized the utility of the disclosed glass-ceramics in extending the useful life of abrasive products, with express reference to grinding wheels utilizing particulate Al.sub.2 O.sub.3 as the abrasive grain. The glass-ceramic materials described in the patent were prepared from precursor glasses consisting essentially, expressed in terms of weight percent on the oxide basis, of 25-65% B.sub.2 O.sub.3, and 20-75% RO, wherein RO consists of at least one divalent metal oxide selected from the group consisting of CaO, SrO, BaO, MnO, and ZnO, the sum of RO+B.sub.2 O.sub.3 comprising at least 55% of the total composition, with the following optional components in the indicated maxima of up to 20% SiO.sub.2, up to 15% F, up to 25% MgO, up to 10% ZrO.sub.2, and up to 40% Al.sub.2 O.sub.3. A further limitation was observed that, when the Al.sub.2 O.sub.3, was greater than 20%, at least 2% F and/or 10% SiO.sub.2 should be included for optimum results.
U.S. Pat. No. 5,256,603 (Andrus et al.) describes the formation of glass bonded, rather than glass-ceramic bonded, ceramic composite articles, with special reference being made to the use of the patented glasses to bond particulate abrasive materials into such articles as grinding wheels. Thus, the patent discusses the preparation of glass bonded ceramic composites wherein the ceramic particulates are selected from the group consisting of Al.sub.2 O.sub.3, cordierite, SiC, Si.sub.3 N.sub.4, TiB.sub.2, TiN, zircon, and ZrO.sub.2. The disclosed composites are stated to be useful also as high performance coatings and as substrates for microelectronic circuitry. The patent, however, repeatedly emphasized the strength of the bonding between the inventive glasses and alumina particles, thereby recommending their utility for bonding those particles into abrasive products such as grinding wheels. The glasses described in the patent consist essentially, expressed in terms of weight percent on the oxide basis, 15-35% CaO, 35-55% B.sub.2 O.sub.3, and 10-35% SiO.sub.2, the sum of CaO+B.sub.2 O.sub.3 +SiO.sub.2 constituting at least 80% of the total composition with the molar ratio CaO:B.sub.2 O.sub.3 &lt;1, and up to 20% total of optional components in the indicated proportions selected from the group consisting of
______________________________________ MgO 0-15 Na.sub.2 O 0-20 ZrO.sub.2 0-10 SrO 0-20 K.sub.2 O 0-20 WO.sub.3 0-10 BaO 0-20 Al.sub.2 O.sub.3 0-20 MoO.sub.3 0-10 Li.sub.2 O 0-15 F 0-8 Nb.sub.2 O.sub.5 0-20 ______________________________________